Remove backport annotations upon conversion

[bbartley]

This issue concerns converted files that have been round-tripped, which results in superfluous annotations being accumulated.

When converting from 2->3, any SBOL3 backport annotations should be removed. Such annotations are redundant and should not be carried over, since the annotations have presumably been converted and directly expressed as SBOL3 in the resulting file

Similarly, when converting from 3->2, the SBOL2 backport annotations should be removed. The annotations aren’t needed
since they are directly expressible in the resulting SBOL2 file.